Although a two-stroke engine has the advantages of a high ratio of output horsepower/engine weight (volume), it does have two drawbacks as a result of its scavenging cycle, i.e., to scavenge the residual gas left in cylinder by means of fresh air taken in; the two drawbacks are
(11) during scavenging process, part of the fresh mixture is susceptible to flowing to the exhaust port, i.e., a well-know expression of "short-circuiting loss of unburned mixture"; in a two-stroke engine, the "short-circuiting loss of un-burned mixture" is related to engine load. During low-load operation, there is not much fresh air entered from the scavenging port, and only a small part of fresh mixture flows to the exhaust port. As soon as the engine load is increasing, more fresh mixture will enter the cylinder, and then the short-circuiting loss will be increased correspondingly. As a result, the carburetor of a two-stroke engine would exhaust a large amount of hydrocarbons during a higher load running.
Recently, many improvements on two-stroke engine have been made in order to reduce the short-circuiting loss of un-burned mixture; in such improvements, the most effective method is the "in-cylinder direct injection" to replace the conventional carburetor in order to have the scavenging stream become a solid air flow; further, a control means is used for accurately controlling the fuel-injection timing so as to prevent the un-burned fuel from escaping to the exhaust port.
(2). The fresh air stream is unable to clear the residual gas in the cylinder; and then a high residual gas ratio remains in the cylinder; such phenomenon usually takes place during the low-load running or the idle running of an engine, and simultaneously there is less fresh air to enter the cylinder through scavenging ports; in that case, there will be less short-circuiting loss as mentioned in (1) above to take place; however, since the small amount of fresh air in the engine is unable to completely scavenge a considerable quantity of residual gas, the fresh air therein would be diluted by the residual gas; since the fresh air does not flow in continuously, the cylinder would have a high ratio of residual gas after the scavenging stroke being over. Such phenomenon widely exists in two-stroke engines no matter using a conventional fuel injection method or a carburetor type whenever an engine running under a low-load condition or an idle condition. Before ignition, the residual gas ratio in the cylinder would affect the combustion efficiency of an engine. Tn case of the residual gas ratio being increased considerably, the probability of the mixture misfiring will be increased. Even if the mixture is ignited, the high ratio of residual gas would reduce the propagation speed of flame to cause a slow burning or partial misfiring condition, and that is why the two-stroke engine usually exhausts a high hydrocarbons (HC) during low-load running or idle running. Such a phenomenon is not due to the fuel short-circuiting fuel, but is a problem caused by high ratio of residual gas in cylinder. Moreover, if a car or a motorcycle is considered to drive in city according to the standard driving cycle of the automobile and motorcycle, for example the ECE cycle in Europe in air pollution regulations; the fact is that a car or motorcycle is usually running under an idle and low-load condition in most driving time, and that is the problem.
In view of the aforesaid problem, some person has developed a method of "direct-injection stratified-charge engine (or D.I.S.C. engine) to solve the unstable combustion problem of an engine under low-load running. The D.I.S.C. method is not a new idea, bat was a study for four-stroke engine about the last ten or 20 years, which may be seen in some publications of the kind. Such idea is based on to let a high amount of fresh air enter the cylinder under low-load running condition so as to scavenge the residual gas in cylinder; simultaneously, a small amount of fuel is to be injected into the cylinder to provide a low-load output. According to such a method, the load of an engine is merely related to the amount of fuel injected therein, while the air flow entered into the cylinder is maintained at a high value. The small amount of fuel injected in the cylinder would cause an over diluted condition, which is not easy to be ignited unless the stratified charge is formed around the spark plug. To obtain such a ignitable charge as desired, the injection system must provide a high pressure and late injection, and at the same time, the air flow inside the cylinder has to be controlled properly; furthermore, a high atomized and high concentrated fuel charge is also required. Unfortunately, the aforesaid requisites for obtaining the stratified charge are rather difficult to control. In the last ten or 20 years the four-stroke D.I.S.C. engine is still unable to overcome and control the aforesaid technical requirements. Although a low-load output can be obtained, the atomization and concentration of fuel spray thereof are still unable to meet the requirements desired; as a result, a high ratio of hydrocarbons in the exhaust stroke is still a problem to be overcome in a D.I.S.C. engine either a two-stroke or four-stroke one.